Combustors on both new and upgraded industrial gas turbines have recently and increasingly utilized Dry Low NOx combustion systems (“DLN combustors” or “DLN combustion systems”). DLN combustors employ lean, premixed combustion for achieving low nitrogen oxide (“NOx”) and carbon monoxide (“CO”) emissions. DLN combustors have also been utilized in order to comply with more strict regulations for pollutant emissions. The cooler flame temperatures of the lean premixed flames of DLN combustors are the primary mechanism for producing lower NOx levels. As a result, DLN combustors have largely replaced standard diffusion combustors that employ water or steam injection for achieving reduction in NOx emissions.
Many features in DLN combustors make them more complex systems to operate and control than standard diffusion combustors. The increased complexity of DLN combustors may therefore impact the operability, flexibility, and reliability of a gas turbine with which the DLN combustor is installed. Thus, operating and maintenance practices, which would be acceptable or have no negative impact with standard diffusion combustors, are not acceptable when DLN combustors are installed.
For example, modern gas turbines equipped with DLN combustion systems suffer the problem of thermo-acoustic combustion instability. The acoustic characteristics of the combustion chamber, as well as the response of the combustion flame to the fluctuations of pressure, all play a fundamental role on the conditions which may occur when DLN combustion systems are affected by combustion instabilities. Indeed, thermo-acoustic interaction between acoustic pressure oscillations and flame heat release fluctuations are often regarded as the main origin of combustion instabilities in gas turbines. These instabilities must be avoided since they may generate structural vibrations that, in some cases, may lead to failure of the system.
Issues in detecting and controlling combustion instability have existed ever since flames have been confined within ducts. These instabilities occur in many types of combustion system from domestic heating systems to rocket motors and gas turbines. In order to detect and prevent these instabilities from occurring, dynamic pressure sensors are often utilized in gas turbine DLN combustion chambers to detect initial combustion instability. If these instabilities are detected at an early stage, the turbine can be adjusted with little effort for smooth and steady combustion.
These dynamic pressure sensors measure and acquire pressure-related environmental data which may be used to confirm proper operational health of the combustion system, and which can also be used to tune the gas turbine engine so that it is operating with an appropriate balance between combustion dynamics and emissions.
In addition to measuring dynamic pressure, the determination of flame temperature has historically been attributed great importance in the field of combustion technology. Flame temperature is directly correlated with the chemical reaction kinetics and the formation of pollutants such as, for example, NOx. Moreover, knowledge of the release of energy during the combustion process is indispensable for the design of combustion chambers and determination of the mechanical and the thermal loads of all components utilized in the combustion system. U.S. Patent Pub. No. 2012/0196234 to Bulat et al., the entire contents of which are herein incorporated by reference, discloses several possible positions for a temperature sensor to be used with a combustion chamber. U.S. Pat. No. 5,828,797 to Minott et al., the entire contents of which are herein incorporated by reference, discloses the continuous optical monitoring of the combustion process within an igniter port or a pilot flame port. Other references which generally disclose the desirability to obtain temperature data as part of the combustion process include U.S. Pat. No. 6,546,556 to Ginter and U.S. Patent Pub. No. 2011/0093182 to Weber et al., the entire contents of each are herein incorporated by reference.